Metallic implants are widely utilized in modern medicine. Metals such as titanium, cobalt chrome, nitinol and stainless steel are widely used as implant materials due to their combination of strength, corrosion resistance and biocompatibility. These metals are commonly found in orthopaedic implants, where they are offered as either cemented or cementless implants, depending on whether a cement is used to hold the implant in place. These implants are routinely roughened to produce a surface onto which osteoblasts can attach and proliferate to promote bone fixation. Early implants achieved this roughening through simple processes such as grit blasting. More modern designs are focused around complex surface geometries based on three dimensional surfaces. For instance, DePuy provides a surface termed Porocoat®, which is derived from sintered metal beads. A further enhancement on this surface is their Gription® surface. Stryker also provides a beaded metal surface and is developing a laser process termed SLM (selective laser melting) to deliver a three dimensional surface. Zimmer have launched a porous metal finish called Trabecular Metal™. Other versions such as plasma sprayed Ti foam are well known in the medical device industry. Although these surfaces are different, they all share a common concept in that they are open, porous three dimensional metal surfaces designed to optimize bone growth and implant fixation.
There remain, however, on-going issues relating to microbial infections with these implants. Infections can be pre-existing, introduced during surgery or can migrate to the implant surface post operatively. Infection can induce bone degeneration that can loosen the implant. As a consequence, expensive, complex and difficult revision surgery with prolonged and extensive antimicrobial agent administration may be necessary.
Numerous attempts have been made to minimize infections through strategies such as adding antibiotics to bone cements. This provides an elution of drugs from the cement, which helps to eliminate microbes in the vicinity of the implant during early stage fixation. Other attempts have focused on trying to attach active agents such as antibiotics to the surface of the metal implant. Simply dipping the metal implant in antibiotic solution can result in a drug elution profile having a burst release of very short duration. Thus, this approach offers limited value. Slow elution has been attempted by entrapping the drugs in a polymer coating on the implant surface and these drug loaded polymeric coatings are well established in medical devices. For example, the drug eluting stents now dominate the stent market and are designed to deliver therapeutic agents over several weeks or months. However, this solution is not applicable to the hard tissue sector, as the presence of the polymer coating on the biocompatible metal implant surface can impede bone fixation. Therefore an alternative strategy is called for.